The present invention relates to a projection type image display device, and particularly relates to projection lenses with short projection distance and wide field of angle and a good cost performance image display device using the projection lenses in which bright projection images are projected even on the corner portion of the screen while the image display device is shaped to a compact form.
In order to enjoy high definition images such as Hivision images, heretofore, display devices of large screen size have been developed. Such display devices of large screen size are classified into two types, namely, direct view type and projection type. The two types of display devices have both merits and demerits. It is generally said that the projection type display devices are superior in weight, depth and cost to the direct view type display devices but inferior in brightness of picture and resolution to the direct view type display devices. With the advance of improvement in projection tubes and key parts such as projection lenses and screens, however, recent projection type display devices are considerably improved with respect to the aforementioned problem. Considering both easiness of setting in home and portability, the projection type display devices are expected to be used widely in the future.
In the process of development of such projection type display devices, there have been attempts to reduce f-number by using a large number of aspherical lenses to thereby secure brightness of picture equal to that of the direct view type display devices, as disclosed in U.S. Pat. Nos. 4,682,862 and 4,792,217. Recently, the case of use of doublet glass lenses in order to attain improvement both in brightness and in focusing performance has been disclosed in U.S. Pat. No. 5,200,814. As a result, projection lenses of about f/1.0 are employed in the projection type television devices at present so that practically sufficient brightness is secured in the center portion of the screen.
Although above description has shown the fact that the present projection type image display devices using a plurality of such large-sized projection lenses are lower in cost than the direct view type display devices if the projection type display devices are equal in size to the direct view type display devices, further reduction in cost is required to attain wide use of the projection type display devices. It is however difficult to attain reduction in cost of the projection lenses because the number of the projection lenses used has been already reduced to 4 or 5, compared with reduction in cost of circuit parts as achieved by large-scale integration.
When the present projection type television devices are compared with the direct view type television devices, shortage of corner brightness of image is particularly prominent in the performance of the projection type television devices at present. It is therefore necessary to increase the corner brightness of projection lenses. The brightness in the corner portion of the screen in the present direct view type television devices is about 60% as the ratio of brightness at an image point of relative image height =0.9 as a representative point to brightness at the center of the screen, whereas the brightness in the projection type television devices is about 30%, that is, the brightness in the projection type television devices is lower than the brightness in the direct view type television devices.
In order to utilize compactness which is the largest merit of the projection type television devices, it is necessary that the projection distance is further shortened. Although the half field of angle in projection lenses of short projection distance is about 35.degree. at present, projection lenses with half field of angle of about 38.degree. are required to attain compact setting such as depth of about 40 cm with respect to the screen size of 40 inches.
As for cost of projection lenses, glass power lenses which are extraordinarily larger in volume than small-sized camera lenses occupy the large weight of the whole cost. It may be therefore considered that low-cost materials are used for glass lenses. However, low-cost materials which are high in index so that monochromatic aberration can be corrected easily, are generally large (low Abbe number) in dispersion to cause chromatic aberration. On the contrary, materials which are low (high Abbe number) in dispersion so that chromatic aberration can be reduced easily, are low in index to make it difficult to correct monochromatic aberration. The long and short of it is that there is no low-cost material balanced on optical design.